Radiation detector

ABSTRACT

In a radiation detector, an active matrix board is formed of switching elements of polycrystalline silicon thin film transistors produced by a poly-silicon (poly-Si) process, charge storage capacitances, insulating layers, electrodes, gate lines and data lines, on which a converting layer is formed by polycrystalline, such as CdTe and CdZnTe, having a high sensitivity with respect to light and radiation at a film-forming temperature higher than 300° C. A gate driving circuit and a signal reading-out circuit are provided on the active matrix board, and signals of the respective images are scanned to take out to the outside. Thus, by using the high heat resistant matrix process board, the radiation detector having a wide dynamic range and a high signal to noise (S/N) ratio can be obtained.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The invention relates to a radiation detector for industrial andmedical purposes, more particularly, a direct-converting-type radiationdetector using a converting layer for absorbing light or radiation togenerate a pair of electron-hole.

[0002] Heretofore, there has been known a two-dimensional radiationimage detector, wherein semiconductor sensors (light or radiationdetecting elements) sensitive to X-rays to generate charge(electron-hole) are disposed two-dimensionally, and electric switchesare provided thereto, respectively. Thus, the electric switches on therespective columns are consecutively turned on and charges of thesensors of the respective rows are read out.

[0003]FIG. 3 is a front view showing a structure of a conventionaltwo-dimensional radiation image detector. FIG. 2 is a sectional viewshowing a structure of one picture element. The conventionaltwo-dimensional radiation image detector includes an active matrix board10 with a glass supporting base plate 11; a converting layer 1 of alight conductive layer formed almost all over the surface of the activematrix board 10; and a common electrode 1 b disposed on an upper portionthereon. The active matrix board 10 includes electrode wirings in amatrix form formed of gate lines 4 and data lines 5 formed on an upperportion thereof with a layer insulating film 2 b therebetween, switchingelements 3 formed of thin film transistors (TFT), charge storagecapacitances (Cs) 2, each being disposed between a capacitance electrode2 a and grounded electrode 2 c, and pixel electrodes 1 a connected tothe capacitance electrodes 2 a, respectively, provided on upper portionsthereof.

[0004] As the converting layer 1 constituting a light conductive layer,a semiconductor material for generating a charge (electron-hole) whenradiation, such as X-rays, is irradiated, is used. More specifically,amorphous selenium (a-Se) having a high dark resistance, a wide dynamicrange with respect to X-ray irradiation, a good signal to noise (S/N)ratio and a good light conductive characteristic, is used. Theconverting layer 1 as the light conductive layer (a-Se) is formed on aglass or quartz base plate, on which the active matrix driving circuitis provided, in a thickness of 300 to 1,000 μm by a vacuum depositionmethod at a temperature lower than 250° C. Also, since it is possible tolower a cost for a large converting layer 1, a thin film transistor filmof hydrogenation amorphous silicon (a-Si:H) containing hydrogen is usedas the semiconductor film for the active matrix driving circuit.

[0005] As described above, the active matrix board is structured by thethin film transistors (TFT) formed of the amorphous silicon (a-Si:H),X-Y matrix electrodes and charge storage capacitances (Cs), so that theactive matrix board has the same structure as that of the active matrixboard to be used for an active matrix type liquid-crystal display device(AMLCD). Thus, it is easy to use the active matrix board (AMLCD) as theactive matrix board 10 for a two-dimensional radiation detector byslightly modifying its design.

[0006] Next, an operation and theory of the two-dimensional radiationpicture image detector having the above structure are explained. Whenradiation is irradiated to the converting layer 1, such as amorphousselenium (a-Se) film, a charge (electron-hole) is generated in theconverting layer 1. Since the converting layer 1 and the charge storagecapacitance (Cs) 2 are electrically connected in series, when a voltageis applied between the common electrode 1 b on an upper part and thecapacitance electrode 2 a, the charges (electron-hole) generated at theconverting layer 1 are moved to a plus electrode side and a minuselectrode side, respectively, so that the charges are stored in thecharge storage capacitance (Cs) 2.

[0007] According to the above operation, when an input signal of a gateline 4 from a gate driving circuit 6 provided to an outer portion isinputted to a thin film transistor (TFT) gate, the thin film transistor(TFT) opens. Then, the charges stored in the charge storage capacitance(Cs) 2 are taken out from the source to the drain, and then taken out toa signal reading circuit 7 provided on an outer portion through the dataline 5. Since the electrode wirings of the gate lines 4 and data lines5, the thin film transistors (TFT) of the switching elements 3 and thecharge storage capacitances (Cs) 2 are disposed in the X-Y matrix form,picture image information of the X-ray can be obtained two-dimensionallythrough the data lines 5 by sequentially scanning the signals inputtedinto the thin film transistors (TFT) gate electrodes from the gate lines4.

[0008] Incidentally, the above-described two-dimensional radiation imagedetector can also be used as a two-dimensional image detector of avisible light and infrared light, in case the converting layer 1 to beused has a light conductivity with respect to not only radiation, suchas X-ray, but also the visible light and infrared light.

[0009] The conventional two-dimensional radiation image detector isstructured as described above, wherein the amorphous selenium (a-Se) asthe converting layer 1 is directly formed on the active matrix board 10by a vapor deposition method. In this structure, there are followingproblems.

[0010] (1) In case semiconductor materials other than amorphous selenium(a-Se) as the converting layer 1 are used, semiconductor materials to beused are restricted due to a heat resistance problem of the activematrix board 10. For example, in case a polycrystalline film of CdTe orCdZnTe having a more improved sensitivity with respect to X-ray whencompared with amorphous selenium is formed by an MOCVD method, proximitysublimation method, paste baking method or the like, which is suitablefor forming a large area film, a film forming temperature higher than300° C. is required. However, generally, a heat resistant temperature ofthe switching element (TFT) 3 formed on the active matrix board 10 isabout 250° C., in case the amorphous silicon (a-Si:H) is used as anormal semiconductor layer. Therefore, there is a difficulty in directlyforming a polycrystalline film of CdTe and CdZnTe on the active matrixboard 10 of a-Si:H.

[0011] (2) In a large two-dimensional image detector, wirings of thegate lines 4 and data lines 5 in the active matrix board 10 become long,and the gate lines 4 and the data lines 5 are connected to the gatedriving circuit 6 and signal reading-out circuit 7 through flexiblepanel circuits (FPC) by using anisotropic conductive films (ACF) and thelike. In this case, there is a problem such that noises are generated bythese parasitic resistance and capacitance component to therebydeteriorate a signal to noise (S/N) ratio and a dynamic range asimportant performances of the two-dimensional image detector.

[0012] The present invention has been made to solve these problems, andan object of the invention is to provide a radiation detector, wherein ahigh thermal resistant matrix process board is used so thatpolycrystalline film of CdTe, CdZnTe and the like can be directly formedthereon, to thereby provide a low signal to noise (S/N) ratio andprevent reduction of a dynamic range caused by connection of circuits.

[0013] Further objects and advantages of the invention will be apparentfrom the following description of the invention.

SUMMARY OF THE INVENTION

[0014] In order to attain the above objects, according to a first aspectof the invention, a radiation detector of the present inventioncomprises an active matrix board including gate lines and data linesarranged in a two-dimensional lattice shape and formed on an insulatingplate, a plurality of high-speed switching elements provided atrespective lattice points and connected to the gate lines and the datalines, picture element electrodes connected to source electrodes of thehigh-speed switching elements, and charge storage capacitances, eachbeing disposed between the capacitance electrode and the ground; and aconverting layer formed on upper parts of the picture element electrodesto generate a pair of electron-hole by absorbing light or radiation,wherein a poly-silicon (Poly-Si) process board is used as the activematrix board.

[0015] According to a second aspect of the present invention, theconverting layer for generating the pair of electron-hole by absorbingthe light or radiation is a polycrystalline film of CdTe or CdZnTe.

[0016] The radiation detector of the present invention is structured asdescribed above, wherein since the active matrix board is formed of apoly-silicon (Poly-Si) process board, the poly-crystalline convertinglayer can be formed at a film-forming temperature higher than 300° C.Therefore, a two-dimensional image detector can be structured by using avariety of polycrystalline semiconductor films, such as CdTe and CdZnTehaving a high sensitivity with respect to light and radiation, as aconverting layer.

[0017] Also, since the active matrix board is formed of a poly-siliconprocess board, signal process circuits, such as a gate driving circuit,a signal reading-out circuit formed of a pre-amplifier and the like canbe provided on a large active matrix board to thereby obtain atwo-dimensional radiation image detector having a low noise and a largedynamic range.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a front view showing a circuit structure of a radiationdetector according to the invention;

[0019]FIG. 2 is a sectional view of the radiation detector forexplaining a manufacturing method of the radiation detector of theinvention and the conventional circuit; and

[0020]FIG. 3 is a front view showing a circuit structure of aconventional radiation detector.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] A radiation detector of an embodiment according to the presentinvention is explained with reference to FIGS. 1 and 2.

[0022]FIG. 1 is a front view of a radiation detector according to theinvention, and FIG. 2 is a sectional view of a picture element of theradiation detector.

[0023] A radiation detector of the present invention comprises an activematrix board 9, a converting layer 1, a common electrode 1 b provided onan upper portion of the converting layer 1, a gate driving circuit 6,and a signal reading-out circuit 7, both being disposed outside a pixelregion 8 of the active matrix board 9. The active matrix board 9includes a high thermal resistant insulating supporting base plate 11,gate lines 4 and data lines 5 arranged in a two-dimensional latticeshape on the supporting base plate 11, high-speed switching elements 3formed of a plurality of polycrystalline silicon thin film transistors(Poly-Si TFT) provided to respective lattice points and connected to thegate lines 4 and the data lines 5, pixel electrodes 1 a connected toelectrodes of sources 3 b of the high-speed switching elements 3,respectively, capacitance electrodes 2 a connected to the pixelelectrodes 1 a, respectively, and charge storage capacitances 2, eachbeing disposed between the capacitance electrode 2 a and groundedelectrode 2 c. The converting layer 1 is formed at upper portions of therespective pixel electrodes 1 a to generate a pair of electron-hole byabsorbing light or radiation.

[0024] The active matrix board 9 is structured such that, first, thegate lines 4 to be connected to the gates 3 a of the thin filmtransistor (TFT) elements, the ground electrodes 2 c and layerinsulating film 2 b provided thereon are formed on the high thermalresistant insulating supporting base plate 11. Then, semiconductorlayers for the switching elements 3 formed of the polycrystallinesilicon thin film transistors (Poly-Si TFT) are formed at the positionsof the gates 3 a. Thereafter, each data line 5 is disposed in a drain 3c of the thin film transistor (TFT) element to form the capacitanceelectrode 2 a to the source 3 b of the thin film transistor (TFT)element. Thus, the charge storage capacitance 2 is formed between theground electrode 2 and capacitance electrode 2 a. Then, the pixelelectrode 1 a connected to the capacitance electrode 2 a is formed on anupper portion of each pixel through an insulation protective film 13.

[0025] In the conventional radiation detector, a thin film transistor(TFT) base plate formed of amorphous silicone (a-Si) is used. However,the radiation detector of the invention is produced by using a thin filmtransistor (TFT) base plate formed of the polycrystalline silicon(Poly-Si). The reason for using the polycrystalline silicon is that thepolycrystalline silicon thin film transistor (Poly-Si TFT) base platehas a heat resistant temperature higher than that of the amorphoussilicon thin film transistor (a-Si TFT) base plate, so that it is stableeven at an environmental temperature higher than 300° C.

[0026] Next, a production process of an Si gate nMOS of thepolycrystalline silicon thin film transistor (Poly-Si TFT) is explained.First, a thin silicon dioxide film is formed on a base plate surface atan initial step and a silicon nitride film is further formed thereon.Then, the silicon nitride film in a region other than the transistorregion on the base plate surface is removed, and an ion implantation fora channel stopping is carried out, which is then subject to a thermaloxidation to grow a field oxide film thicker than 500 nm on the portionwhere the silicon nitride film is removed. At this time, a half of thethick oxide film is implanted into the base plate. Next, the oxide filmand nitride film are removed, and again, a thin gate oxide film of theorder of 20 to 40 nm is formed or grown. The thin oxide film is removed,and n⁺ ion is applied thereon with polycrystalline silicon as a mask toform the source and drain regions. Next, a thick oxide film or PSG isformed by a chemical vapor deposition (CVD) method, a hole for a contacthole is made, an aluminum film is formed thereon by a sputtering method,and electrodes for the source and drain are formed by patterning.

[0027] In the radiation detector of the invention, the active matrixboard 9 is produced in the same process as that of a poly-silicon(Poly-Si) active matrix board formed in a process for manufacturing aliquid-crystal display device. An entire area of a two-dimensional imagedetector is about 50 cm×50 cm, pixels are arranged in a matrix shapewith a pitch of 150 μm. Further, a gate driving circuit 6 for sendingsignals to the gates 3 a of the thin film transistor elements (switchingelements 3 of p-Si) formed on the active matrix board 9, and signalreading circuit 7 including a pre-amplifier circuit for taking thereincharge signals of the charge storage capacitances 2 from the convertinglayer 1 and the like are provided in circumferential portions of theactive matrix board 9.

[0028] Also, the surface of the poly-silicon (poly-Si) active matrixboard 9 is covered by the silicon nitride (SiN) film and an insulationprotective film 12 of an organic-series resin except for the pixelelectrodes la corresponding to the pixels. Then, after portions wherethe films are not required in the circumferential portion and the likeare masked, and the active matrix board 9 is set on a film formingdevice. A polycrystalline film of CdTe, CdZnTe or the like for theconverting layer 1 is formed by an MOCVD method, a proximity sublimationmethod, a paste baking method or the like, which are suitable forforming a film for a wide area. If necessary, a charge blocking layer ofCdS, ZnTe or the like may be provided on and under the converting layer1. Thereafter, on an upper portion of the converting layer 1, there isformed the common electrode 1 b for transferring a charge(electron-hole) generated at the converting layer 1 to a plus electrodeside and minus electrode side, and applying a bias voltage to collect tothe charge storage capacitance (Cs) 2. Further, if necessary, a surfaceprotective film may be formed.

[0029] In the radiation detector according to the present invention,since the switching element 3 formed of the poly-crystalline silicon(poly-Si) is formed on the active matrix board 9, the semiconductorfilm, such as CdTe and CdZnTe, of the converting layer 1 can be formedunder a film forming condition of a temperature higher than 300° C.because the heat resistant temperature of the present active matrixboard 9 is high, while the heat resistant temperature is about 250° C.in the conventional amorphous silicon switching element.

[0030] Then, for the connection of the gate lines 4, data lines 5,common electrode 1 b and grounded electrodes 2 c formed on the activematrix board 9 having a wide area, with the gate driving circuit 6 andthe signal reading-out circuit 7 provided at the circumferential portionoutside the active matrix board 9, a signal process circuit can be madesimultaneously on the same active matrix board 9. Therefore, there aregenerated no noises caused by parasitic resistance and capacitancecomponents through connection of FPC or the like to thereby obtain atwo-dimensional radiation image detector having a low noise and a largedynamic range.

[0031] In the above embodiment, the radiation detector employing theactive matrix board 9 produced by a poly-silicon (poly-Si) process isexplained to be used for detecting light or radiation. However, theradiation detector can also be applied to the two-dimensional pictureimage detector of ultraviolet ray, infrared ray or the like.

[0032] Also, as materials for the semiconductor film to be used for theconverting layer 1, CdTe and CdZnTe have been explained. However,materials necessary for obtaining desired detecting characteristics canbe selected, if a film can be formed within a range of the thermalresistant temperature of the poly-silicon active matrix board.

[0033] The radiation detector according to the present invention isstructured as described above, and the active matrix board formed of thepolycrystalline silicon thin film transistors (poly-Si TFT), insulatinglayers and electrodes produced by the poly-silicon (poly-Si) process isused. Thus, a stable polycrystalline converting layer can be obtainedeven at a film-forming temperature higher than 300° C. Accordingly, avariety of polycrystalline semiconductor films, such as CdTe and CdZnTe,having a high sensitivity with respect to light and radiation can beused to thereby obtain a radiation detector having a good signal tonoise (S/N) ratio and a wide dynamic range.

[0034] Further, since the signal process circuits are formedsimultaneously on the same active matrix board 9, there are generated nonoises by the parasitic resistance and capacitance components throughconnection of FPC and the like to thereby obtain good picture images.

[0035] While the invention has been explained with reference to thespecific embodiments of the invention, the explanation is illustrativeand the invention is limited only by the appended claims.

What is claimed is:
 1. A radiation detector comprising: an active matrixboard including gate lines and data lines arranged in a two-dimensionallattice form, a plurality of high-speed switching elements provided atrespective lattice points and connected to the gate lines and the datalines, each having a source electrode, pixel electrodes connected to thesource electrodes of the high-speed switching elements, and chargestorage capacitances, each being disposed between the pixel electrodeand a ground electrode, said active matrix board being formed of apoly-silicon process board; and a converting layer formed on the pixelelectrodes to generate a pair of electron-hole by absorbing one of lightand radiation.
 2. A radiation detector according to claim 1 , whereinsaid converting layer for generating the pair of electron-hole byabsorbing one of light and radiation is a polycrystalline film of one ofCdTe and CdZnTe.
 3. A radiation detector according to claim 1 , whereinsaid high-speed switching elements are formed of polycrystalline siliconthin film transistors.
 4. A radiation detector according to claim 3 ,wherein said active matrix board further includes a base plate havinghigh heat resistance and insulating property, an insulating filmdisposed on the base plate and sandwiched by the gate lines and datalines, an insulating protecting layer disposed on the insulating filmabove the switching element, and a common electrode disposed on theconverting layer.
 5. A radiation detector according to claim 4 , furthercomprising gate driving circuit to be connected to the gate lines, asignal driving circuit to be connected to the data lines, and a signalprocess circuit formed on the active matrix board for connecting thegate lines and data lines to the gate driving circuit and the signalprocess circuit.